How to make node disappear
First they use a Node Wrangler shortcut.  They're doing a Shift +  RMB and drawing a line over the two noodles coming out of the Value node.
This command combines the two noodles into a Reroute node that splits out to its two destinations.
Then they delete the Value node and bring a new input over from the Group Input.  Essentially they've recreated the Value node as a slider on the node group.